Stock-Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	$ 76.4	$ 26.9	$ 25.2
Stock Options [Member] \| U.K. Save As You Earn (SAYE) Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	$ 1.2	$ 1.3	$ 1.4
Number of shares available for grant (in shares)	271,000
Other Stock Compensation Programs [Abstract]
Options granted (in shares)	0	0	85,000
Exercise price - options granted (in dollars per share)	$ 0	$ 0	$ 90.97
Options exercised (in shares)	17,000	19,000	28,000
Weighted average exercise price (in dollars per share)	$ 160.41	$ 142.96	$ 121.70
Performance Shares [Member]
Restricted stock unit activity [Roll Forward]
Unvested at beginning of period (in shares)	93,100	0	0
Granted (in shares)	196,500	100,600	0
Vested (in shares)	0	0	0
Forfeited (in shares)	(2,800)	(7,500)	0
Unvested at end of period (in shares)	286,800	93,100	0
Share-based Payment Arrangement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant (in shares)	1,400,000
Restricted stock unit activity [Roll Forward]
Unvested at beginning of period (in shares)	652,700	727,700	750,900
Granted (in shares)	1,494,500	277,200	188,400
Vested (in shares)	(287,700)	(292,500)	(186,500)
Forfeited (in shares)	(40,400)	(59,700)	(25,100)
Unvested at end of period (in shares)	1,819,100	652,700	727,700
Restricted stock unit activity, additional disclosures [Abstract]
Weighted average grant date fair value, beginning of period (in dollars per share)	$ 131.32	$ 118.96	$ 113.97
Weighted average grant date fair value, granted (in dollars per share)	141.06	156.13	119.08
Weighted average grant date fair value, vested (in dollars per share)	116.32	125.01	99.23
Weighted average grant date fair value, forfeitures (in dollars per share)	137.49	131.84	117.07
Weighted average grant date fair value, end of period (in dollars per share)	$ 141.51	$ 131.32	$ 118.96
Weighted average remaining contractual life, unvested shares outstanding and expected to vest	2 years 4 months 20 days	2 years 7 days	1 year 2 months 26 days
Unamortized deferred compensation cost	$ 134.9
Fair value of shares vested	33.5	$ 43.3	$ 18.5
Grant date fair value	210.8	36.5	22.4
Share-based Payment Arrangement [Member] \| Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	13.6	3.2	0.0
Share-based Payment Arrangement [Member] \| Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Payment Arrangement, Expense	$ 61.6	$ 22.4	$ 23.8
Stock Options [Member] \| Jones Lang Lasalle Savings Related Share Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Discount on purchase price (in hundredths)	15.00%
Other Stock Compensation Programs [Abstract]
Options outstanding (in shares)	69,000	97,000	133,000
Restricted Stock Units (RSUs) [Member]
Restricted stock unit activity [Roll Forward]
Unvested at beginning of period (in shares)	559,600	727,700	750,900
Granted (in shares)	1,298,000.0	176,600	188,400
Vested (in shares)	(287,700)	(292,500)	(186,500)
Forfeited (in shares)	(37,600)	(52,200)	(25,100)
Unvested at end of period (in shares)	1,532,300	559,600	727,700
Vesting Period One [Member] \| Stock Options [Member] \| Jones Lang Lasalle Savings Related Share Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Vesting Period Two [Member] \| Stock Options [Member] \| Jones Lang Lasalle Savings Related Share Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	5 years
HFF Acquisition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Granted to Employees in Conjunction with Acquisition	1,111,300
Shares Issued to Replace Legacy Awards	474,300
Shares Issued for Retention of Employees	637,000.0
HFF Acquisition [Member] \| Share-based Payment Arrangement [Member]
Restricted stock unit activity, additional disclosures [Abstract]
Unamortized deferred compensation cost	$ 84.0

-- None. No documentation exists for this element. --

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